The presence of hydrogen cyanide (HCN) in various gaseous streams complicates removal of additional impurities, e.g., removal of H.sub.2 S or CO.sub.2, and poses problems insofar as product quality and pollution control requirements are concerned. In particular, gas streams derived from the gasification of coal generally have significant minor quantities of HCN which must be dealt with before the gas is utilized.
Accordingly, a practical and efficient procedure for removing impurity HCN might have great economic importance. The invention is such a process.
In my copending application entitled HCN Removal, U.S. Ser. No. 556,257 filed even date herewith, there is described a process in which an HCN-containing gaseous stream is contacted with an ammonium polysulfide solution to form ammonium thiocyanate, at least a portion of the ammonium thiocyanate solution is removed from the contacting zone and the pH thereof is lowered to precipitate sulfur and release H.sub.2 S, and the remaining solution is subjected to hydrolysis conditions to produce H.sub.2 S, NH.sub.3, and CO.sub.2. A significant aspect of the invention in that application is the removal of sulfur prior to hydrolysis. The present invention also provides an efficient manner of sulfur removal.